sea; fishes of long extinct species dart with sudden gleam through its middle depths; and far below, on its white floor, the sea urchin creeps, and the spatangus burrows, and crania and terebratulæ have cast anchor, and the Crista Galli (or carinated oyster) opens its curiously plicated valves, carved with the zigzag mouldings of a Norman doorway, and the flower-like marsupite expands its living petals. And, dim and distant in the direction of the future Grampians, we may espy a cloud-enveloped island; but such is its remoteness, and such the enveloping haze, that we can know little more than that it bears along its shores and on its middle heights a forest of nameless trees, unchronicled by the fossil botanist.

In bringing to a close this part of my subject, let me here remark, that, if we except the obscure and humbly organized diatomaceæ, -a microscopic family of organisms which some of our authorities deem animal and some vegetable, and of which hundreds and thousands would find ample room in a single drop of water, we have now reached a point in the history of our country, in which there existed no species of plant or animal that exists at the present time. Not a reptile, fish, mollusc, or zoöphyte of the Cretaceous system continues to live. We know that it is appointed for all individuals once to die, whatever their tribe or family, because hitherto all individuals have died; and Geology, by extending our experience, shows us that the same fate awaits on species as on the individuals that compose them. In the one case, too, as in the other, death has its special laws; but the laws which determine the life and death of species seem widely different from those which regulate the life and death of individuals and generations. In general, and with but a few exceptions in favor of the cold-blooded division of the

vertebrata, the higher order of animals live longest. A man may survive for a hundred years; an ephemera bursts from its shell in the morning, and dies at night. But it is far otherwise with the higher orders of species. Molluscs and corals outlive the vertebrata; and tribes of the low infusory animals outlive molluscs and corals. We know not that a single shell of at least the latter Pleistocene period has become extinct; but many of its noblest quadrupeds, such as the Irish elk, the cave-bear, tiger, and hyena, and the northern rhinoceros, hippopotamus, and elephant, exist no longer. And as we rise into the remote past, and take farewell, one after one, of even the lower forms, shells and corals, and get into a formation all of whose visible organisms are old-fashioned and extinct, we apply the microscope to its impalpable dust, and again, among still humbler and lowlier shapes, find ourselves in the presence of the familiar and the recent. In another sense than that which the old poet contemplated, we learn from the history of species that the most lowly are the most safe.

"The tallest pines feel most the power

Of wintry blasts: the loftiest tower

Comes heaviest to the ground.

The bolts that spare the mountain side

His cloud-cap't eminence divide,

And spread the ruin round."

How long some of these extinct species may have lived we know not, and may never know; but in all cases their term of existence must have been very extended. Even the extinct elephant lived long enough as a species to whiten the plains of Siberia with huge bones, and to form. quarries of ivory that have furnished the ivory market for year after year with its largest supplies. And of some of the humbler species of animals, the period during which

they have continued to live must have been vastly more protracted. Cyprina Islandica seems to have come into existence at least as early as the fossil elephant; and now, thousands of years after the boreal pachyderm is gone, the boreal shell still exists by millions, and evinces no symptom of decline. And yet, since the commencement of the great Tertiary division, series of shells, as hardy, apparently, as Cyprina, have in succession come into being, and then ceased to be. The period over which we have passed includes generations of species. But there was space enough for them all in the bygone eternity. It has sometimes appeared to me as if, from our own weak inability to conceive of the upper reaches of that awful tide of continuity which had no beginning, and of which the measured shreds and fragments constitute time, we had become jealous lest even God Himself should have wrought in it during other than a brief and limited space, with which our small faculties could easily grapple.

"Oh, who can strive

To comprehend the vast, the awful truth
Of the eternity that hath gone by,
And not recoil from the dismaying sense
Of human impotence! The life of man
Is summed in birthdays and in sepulchres,
But the eternal God had no beginning."

There are two great infinites, the infinite in space and the infinite in time. It were well, surely, to be humble enough to acknowledge it accordant to all analogy, that as He who inhabits eternity has filled the one limitless void- that of space with world upon world and system upon system, far beyond the reach of human ken, He should also have wrought in the other limitless world -that of time-for age after age, and period after period, far beyond the reach of human conception.

LECTURE FOURTH.

The Continuity of Existence twice broken in Geological History - The Three great Geological Divisions representative of three independent Orders of Existences-Origin of the Wealden in England-Its great Depth and High Antiquity-The question whether the Weald Formation belongs to the Cretaceous or the Oolitic System determined in favor of the latter by its Position in Scotland Its Organisms, consisting of both Salt and Fresh Water Animals, indicative of its Fluviatile Origin, but in proximity to the Ocean-The Outliers of the Weald in Morayshire - Their Organisms - The Sabbath-Stone of the Northumberland Coal Pits-Origin of its Name- The Framework of Scotland - The Conditions under which it may have been formed - The Lias and the Oolite produced by the last great Upheaval of its Northern MountainsThe Line of Elevation of the Lowland Counties - Localities of the Oolitic Deposits of Scotland-Its Flora and Fauna - History of one of its Pine Trees Its Animal Organisms - A Walk into the Wilds of the Oolite Hills of Sutherland.

THE mystic thread, with its three strands of black, white, and gray, spun by the sybil in "Guy Mannering," formed, she said, a "full hank, but not a haill ane:" the lengthened tale of years which it symbolized "was thrice broken and thrice to asp." I have sometimes thought of that wonderfully mingled and variously colored thread of existence which descends from the earliest periods known to the geologist down to our own times, as not unaptly represented by that produced on this occasion from the spindle of the gipsy. We find, in its general tissue, species interlaced with and laying hold of species, as, in the thread, fibre is interlaced with and lays hold of fibre; and as by this arrangement the fibres, though not themselves continuous, but of very limited length, form a continuous cord, so

species of limited duration, that at certain parts in the course of time began to be, and at certain other parts became extinct, form throughout immensely extended periods a continuous cord of existence. New species had come into being ere the old ones dropped away and disappeared; and there occurred for long ages no break or hiatus in the course, just as in the human family there occurs no abrupt break or hiatus, from the circumstance that new generations come upon the stage ere the old ones make their final exit. But in the geological thread, as in that of the sybil, the continuity is twice abruptly broken, and the thread itself divided, in consequence, into three parts. It is continuous from the present time up to the commencement of the Tertiary period; and then so abrupt a break occurs, that, with the exception of the microscopic diatomaceæ, to which I last evening referred, and of one shell and one coral, not a single species crosses the gap. On its farther or remoter side, however, where the Secondary division closes, the intermingling of species again begins, and runs on till the commencement of this great Secondary division; and then, just where the Palæozoic division closes, we find another abrupt break, crossed, if crossed at all,- for there still exists some doubt on the subject, —by but two species of plant.1 And then, from the farther side of this second gap the thread of being continues unbroken, until we find it terminating with the first beginnings of life upon our planet. Why these strange gaps should occur,-why the long descending cord of organic existence should be thus mysteriously broken in three,

we know not yet, and never may; but,

1 For a reference to the research of the last two years, which has been busily at work upon this precise epoch, see Preface.